The Green Dot speed concept
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The Green Dot speed concept
hello there,
I just came across a term called Green dot speed..
According to airbus it is the speed which gives the maximum Lift to Drag ratio (L/D ratio) in flight at a given weight..
'' which is the speed called Vmd ''
as per the theory Vmd drag is the lowest (4deg AOA),we use it during holding over a fix(Maximum endurance speed) and a speed which gives Minimum rate of descent and maximum rate of climb...
and also according to the books
its
Vmd = Maximum endurance,max climb rate
1.32 Vmd = Maximum Range
so basically which is the ''Green dot" speed in the PFD
as in airbus ... Vmd or the 1.32Vmd
Please clear my doubt
thank you in advance
I just came across a term called Green dot speed..
According to airbus it is the speed which gives the maximum Lift to Drag ratio (L/D ratio) in flight at a given weight..
'' which is the speed called Vmd ''
as per the theory Vmd drag is the lowest (4deg AOA),we use it during holding over a fix(Maximum endurance speed) and a speed which gives Minimum rate of descent and maximum rate of climb...
and also according to the books
its
Vmd = Maximum endurance,max climb rate
1.32 Vmd = Maximum Range
so basically which is the ''Green dot" speed in the PFD
as in airbus ... Vmd or the 1.32Vmd
Please clear my doubt
thank you in advance
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Afraid it may not help much but, cut and paste straight from A380 FCOM
GREEN DOT DEFINITION
Green dot is an optimized speed. It is an approximation of the best lift to drag ratio.
Flying at green dot speed provides the best climb performance.
Green dot is:
The final takeoff speed
The target speed in managed mode, when in CONF 0, when the approach phase of the FMS is active
The drift down speed, in case of an engine failure.
DISPLAY
Green dot speed is displayed on the airspeed scale of the PFD.
Green dot speed is displayed inflight, when the FLAPS lever is at 0.
COMPUTATION
Green dot speed is computed by the PRIMs.
It is a function of:
The aircraft altitude
The aircraft gross weight
Note
The FMS also computes the green dot speed. It is displayed on the T. O, APPR, and GA panels of the FMS PERF page.
Green dot is an optimized speed. It is an approximation of the best lift to drag ratio.
Flying at green dot speed provides the best climb performance.
Green dot is:
The final takeoff speed
The target speed in managed mode, when in CONF 0, when the approach phase of the FMS is active
The drift down speed, in case of an engine failure.
DISPLAY
Green dot speed is displayed on the airspeed scale of the PFD.
Green dot speed is displayed inflight, when the FLAPS lever is at 0.
COMPUTATION
Green dot speed is computed by the PRIMs.
It is a function of:
The aircraft altitude
The aircraft gross weight
Note
The FMS also computes the green dot speed. It is displayed on the T. O, APPR, and GA panels of the FMS PERF page.
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Green Dot is:
1. Best lift-to-drag ratio speed (Vmd). In 2 EO out condition, best "glide speed".
2. Maximum gradent of climb speed (max angle).
3. Maximum endurance speed (as pointed out by African Dude, NOT to be confused with Max Range, that is CI 0).
Max Rate Cimb Speed is any speed between Green Dot and Econ Speed.
1. Best lift-to-drag ratio speed (Vmd). In 2 EO out condition, best "glide speed".
2. Maximum gradent of climb speed (max angle).
3. Maximum endurance speed (as pointed out by African Dude, NOT to be confused with Max Range, that is CI 0).
Max Rate Cimb Speed is any speed between Green Dot and Econ Speed.
And as an aside, you can mathematically approximate green dot inflight.
For my Airbus, it's double the gw plus 85, plus 1 kt per 1000 feet above 20k. For other Airbuses the maths would be similar.
For my Airbus, it's double the gw plus 85, plus 1 kt per 1000 feet above 20k. For other Airbuses the maths would be similar.
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And as an aside, you can mathematically approximate green dot inflight.
Max Rate Cimb Speed is any speed between Green Dot and Econ Speed.
Green-dot is Vmd or Vmax angle/Vx and best L/D ratio. BUT calculated for one engine out in Airbus SA a/c. All eng oper speed would be different and is not presented to the flightcrew.
compressor stall's specific formula applies to the A320 (A321 has a different wing). He will also know that – because his formula, as OPEN DES points out, is the precise one used by the FMC (GW in tonnes, by the way) – it also provides a gross-error check of the loadsheet ZFW if compared with the green-dot symbols on the two PFD-ASIs. Each of these is calculated by the associated FAC, which observes the AoA, CAS, and normal G (I think); and uses them to calculate the actual GW.
We used to reckon that a discrepancy of up to 10kt (5 tonnes) was acceptable. Needless to say, the green-dot speed displayed by the FACs on the ASIs was almost invariably higher than the one calculated by the FMC, which relies on the loadsheet ZFW entered by the crew.
We used to reckon that a discrepancy of up to 10kt (5 tonnes) was acceptable. Needless to say, the green-dot speed displayed by the FACs on the ASIs was almost invariably higher than the one calculated by the FMC, which relies on the loadsheet ZFW entered by the crew.
